We found two cases of HIV-1 acute infection, confirmed by nucleic amplification test (NAT) and/or RTPCR, with HIV-1 antibody negative by immunochromatography (IC) method but weakly positive by particle agglutination (PA) test. These cases suggested that IC method was less sensitive than PA test in the detection of acute infections. It is necessary to execute the post counseling that considers the possibility of the acute infection in public health centers and testing places where IC method is used for the screening test. It is also important to recommend taking the following re-examination after a certain period to a person who seems to have had a chance of infection in a short time before testing.